Polymeric membranes for separating mixtures of gases, such as methane and carbon dioxide are known. For example, U.S. Pat. Nos. 7,247,191; 6,932,859; and 6,755,900, which documents are incorporated by reference herein in their entireties, teach crosslinkable polymers and crosslinked hollow fiber membranes made from such crosslinkable polymers. These patents particularly describe a crosslinkable polyimide polymer. The crosslinkable polyimide polymer can be made by monoesterifying a polyimide polymer with a crosslinking agent.
A crosslinked hollow fiber membrane can be made by forming fibers from the crosslinkable polyimide polymer and transesterifying the crosslinkable polyimide polymer within the fibers. More specifically, the crosslinkable polyimide polymer can be formed into crosslinkable fibers, which are then subjected to transesterification conditions in order to create covalent ester crosslinks between the crosslinkable polyimide polymer within the fibers. Such fibers can be hollow fibers or other types of fibers. Crosslinked hollow fiber membranes can be incorporated into a separation module. Other types of membranes for separation include flat sheet separation membranes or flat stack permeators.
Separation modules utilizing hollow fiber membranes include a larger surface area than separation modules utilizing flat sheet or flat stack permeators. Therefore, hollow fiber separation modules have significant separation capability even in a reasonably compact size module. Module size is important in implementing separation modules on offshore platforms, where space and weight are at a premium, to separate mixtures of gases from hydrocarbon producing wells.
The crosslinked hollow fiber membranes have good permeability and selectivity. The crosslinked hollow fiber membranes also have good resistance to plasticization. Plasticization occurs when one or more components of a fluid mixture causes the polymer to swell thereby altering the properties of the membrane. For example, polyimides are particularly susceptible to plasticization by carbon dioxide. Subjecting the fibers to transesterification conditions to crosslink the crosslinkable polyimide polymer within the fibers increases both resistance to plasticization and selectivity.
The above referenced patents recommend that crosslinkable polyimide polymers having an average molecular weight that is not too high or too low be used to make the crosslinked hollow fiber membranes. They further state that the molecular weight of the polyimide polymer is degraded during the monoesterification process. Thus, they recommend use of sufficiently high molecular weight polyimide polymers to accommodate for molecular weight loss during the monoesterification process. However, it is difficult to produce crosslinkable polyimide polymers having such a high molecular weight.
Therefore, there is a need for a method of making a crosslinkable (i.e. monoesterified) polyimide polymer that reduces or eliminates the loss of molecular weight during the monoesterification process. In other words, there is a need for a method of making a high molecular weight, monoesterified polyimide polymer. There is also a need for a method of making a monoesterified polyimide polymer having improved strength, flexibility, and/or spinnability. Further, there is a need for a method of making separation membranes having improved selectivity and permeability.